Visual System: Anatomy and Physiology

Question 1

What is spatial frequency?

Answer 1

cyclical changes in space (high or low)
• More luminance changes per unit length
• Something is happening more “frequently”

e. g.
high: IIIIIII
low: I I I I

Question 2

What are sine wave demonstrations of spatial frequency called?

Answer 2

Sine wave GRATINGS

Question 3

Fourier analysis

Answer 3

All images can be mathematically considered as a summation of sine waves of different spatial frequencies

*example: adding higher frequencies to create square-waves

Question 4

What is a fourier decomposition/transformation?

Answer 4

tells us the low, medium, and high frequency wave components of an image

e.g. *inverse fourier transformation: removing the high frequency components to see the low frequency components

Question 5

How are spatial frequencies related to our limits of acuity?

Answer 5

the size of our cones (S/M/L) determines whether they can be fully activated or inhibited within the spaces of a sine wave

example: when not small enough to match a high SF wave, cones are both inhibited and uninhibited, and we perceive light coming from everywhere (undifferentiated)

Question 6

What is spatial frequency selectivity in ganglion cells?

Answer 6

neurons (ganglion cells) are selective/tuned to specific mediums of spatial frequencies; will not fire if exposed to a higher or lower SF

Question 7

What is phase sensitivity/selectivity of ganglion cells?

Answer 7

whether the neuron fires (above/below baseline) depends on the exact position of the stimulus relative to the receptive field

*SF remains the same, just phase or location changes

Question 8

What is topographic mapping?

Answer 8

Things that are adjacent in the world are represented by adjacent neurons in the thalamus

*also occurs in the cortex

Question 9

What do the first two levels of the LGN correspond to?

Answer 9

Magnocellular layers

associated with signals from rods, responsible for motion and spatial location, and dorsal stream of the visual pathway

• larger cells
• receive input from M ganglion cells

Question 10

What do the bottom four levels of the LGN correspond to?

Answer 10

Parvocellular layers

associated with signals from cones, responsible for acuity; ventral stream of visual pathway

• smaller cells
• receive input from P ganglion cells

Question 11

What are in between the 6 layers of the LGN?

Answer 11

koniocellular layers

Question 12

Which layers of the LGN are received from the same side, and which layers are received from opposite sides?

Answer 12

Ipsilateral eye (same-side): layers 2,3, and 5

Contralateral eye (opposite-side): layers 1,4, and 6

Question 13

What are the different names used for the primary visual cortex?

Answer 13

• striate cortex
• V1
• primary visual cortex

Question 14

What is the cortical magnification factor of V1?

Answer 14

foveal representations are ʻexpandedʼ relative to peripheral representations

Question 15

What are the three V1 cells?

Answer 15

1) Simple cells:
2) Complex cells:
3) End-stop cells:
* all are SF and orientation selective

Question 16

Simple Cell: orientation, phase, motion, and length sensitivity

Answer 16

Orientation: selective
Phase: sensitive
Motion: do not require motion
Length: not selective

Question 17

Complex Cell: orientation, phase, motion, and length sensitivity

Answer 17

Orientation: selective
Phase: insensitive
Motion: sensitive (require motion, sometimes 1 direction)
Length: not selective

Question 18

End-Stop Cell: orientation, phase, motion, and length sensitivity

Answer 18

Orientation: selective
Phase: insensitive
Motion: direction sensitive (require motion in 1 direction)
Length: selective (encode corner; lines STOP)

Question 19

What is orientation selectivity?

Answer 19

depending on the orientation of the object (horizontal vs vertical) the V1 cell will or will-not fire

*unique to V1 cells; not present in retina/thalamus cells

Question 20

What are the two types of simple cells?

Answer 20

Edge detectors:
2 subfields (+)(-)

Stripe detectors:
3 subfields (+)(-)(+)

Question 21

How are complex and end-stop cells different?

Answer 21

End-stop cells are length-specific

• note: otherwise they are very similar: orientation specific, phase non-specific, motion specific, direction specific (although only sometimes in complex)

Question 22

What are columns?

Answer 22

a vertical arrangement of neurons running through the striate cortex; neurons within a single column tend to have similar receptive fields and orientation preferences

• this is how V1 is organized

Question 23

What are the different types of columns?

Answer 23

• location columns
• orientation columns
• ocular dominance columns

Question 24

Location columns

Answer 24

• columns of neurons that all have their receptive field on roughly the same location on the retina
• run perpendicular to the cortex

Question 25

Orientation columns

Answer 25

• column of cells that all prefer the same orientation
• aligned perpendicular to cortex (like location columns)

*different perpendicular penetrations prefer different orientations

Question 26

Ocular dominance columns

Answer 26

a column that prefers input for either the left eye or the right eye

Question 27

Hypercolumns

Answer 27

a block of striate cortex containing at least two sets of columns, each covering every possible orientation (0–180 degrees), with one ocular dominance column preferring each eye

simpler words: a bundle of adjacent location columns that vary in orientation and eye-selectivity

• alternate R/L
• all are encoding the same part of the world
• these columns are how V1 is organized

Question 28

How is contrast defined in a stimulus?

Answer 28

how high or low the amplitude is

• high amplitude = high contrast
• low amplitude = low contrast

Question 29

What is a Contrast Sensitivity Function (CSF)?

Answer 29

the threshold of contrast that is necessary to detect a stimulus; changes depending on SF

*as spatial frequency increases, contrast sensitivity must also increase

Question 30

Where are the two possible streams for visual information to go after V1?

Answer 30

Ventral Stream: “what” system; temporal lobe

Dorsal Stream: “where/how” system; parietal lobe

Question 31

What are the levels of the extra-striate cortex?

Answer 31

V2: illusory contours
V3: donʼt really know - possibly combining
information to construct 3D percepts
V4: color
MT (Medial Temporal): motion
IT (Inferior Temporal): shapes, form, faces

Question 32

What are elaborate cells?

Answer 32

very specific cells that respond selectively to complex forms

*e.g. faces are extremely complex, Fusiform Face Area (FFA) is a structure in IT/temporal lobe that responds specifically to faces